Behavior Of Diaphragm Walls In Soft Clays And Reliablity Analysis

With rapid economic development, the number of braced excavation projects has grown in Singapore and other coastal cities. The technology of diaphragm wall is widely used in these areas where soil deposits comprise of thick soft clay layers overlying stiff clay. However, the traditional empirical methods for predicting wall deflection, ground movement, and strut forces for these excavations are mostly based on excavations with flexible walls. This project studies the excavation behavior using the Finite element method. Uncertainties in the process of design and construction of excavations have been considered through reliability analysis. This project aims to study the behavior of excavations with diaphragm wall in soft clay and to develop a procedure to estimate the reliability index associated with wall and ground movement for excavation system. The following are the major aspects of this project.1. The small strain effect on excavation problems has been assessed using the HSS (Hardening small-strain) model implemented in PLAXIS 8.5 (commercial software). The characteristic of small strain behavior is reflected by analysis of several case histories with the HSS model and comparisons of the behavior with other soil models. A coefficient is introduced to evaluate the small strain effect on wall deflections.2. Parametric studies have been carried out to study the excavation behavior with respect to wall deflection, soil settlement and apparent pressure. The response surfaces equations for predicting the maximum wall deflection and soil settlement have been proposed. A chart is proposed to estimate the maximum apparent pressure for calculating the strut forces.3. Reliability analyses on wall deflection and surface settlement have been illustrated by two examples. Simplified charts have been proposed to assess the failure probability of wall deflection and surface settlement.